Amazonsaurus

Whitlock, John A., 2011, A phylogenetic analysis of Diplodocoidea (Saurischia: Sauropoda), Zoological Journal of the Linnean Society 161 (4), pp. 872-915 : 886

AMAZONSAURUS

The recovery of Amazonsaurus as a basal diplodocoid contrasts with the position of that taxon in previous analyses as basal to Flagellicaudata ( Salgado et al., 2004), in a four-way polytomy with rebbachisaurids ( Salgado et al., 2006), or within Macronaria ( Rauhut et al., 2005). In this analysis, Amazonsaurus shares three unambiguous synapomorphies with the clade Amphicoelias + more derived diplodocoids (MDD): petal shaped anterior caudal neural spines (character 127; reversed in Diplodocidae ); anterior neural spines broader than anteroposteriorly long (character 128); and spinoprezygapophyseal laminae (sprl) present on anterior caudal neural spines (character 138). It cannot be scored for any of the synapomorphies reported here for the clades Rebbachisauridae , Rebbachisaurus + MDD, or Limaysaurinae + Nigersaurinae. Amazonsaurus is excluded from the clade Amphicoelias + MDD by its proportionally short dorsal neural spines (character 118). It is excluded from the clade Rebbachisauridae + Flagellicaudata by its proportionally short mid-caudal vertebrae (character 145); it is unscorable for all other synapomorphies uniting that clade. The proportional height of the neural spine is estimated from the neural spine MN 4558-V and the dorsal centrum MN 4558-V. Although the two were not recovered in association, the height of the neural spine and the length of the centrum are nearly equal. It is highly unlikely then that either derived state (neural spine either twice or four times the centrum length) is present.

Amazonsaurus was united with Flagellicaudata in the analysis of Salgado et al. (2004), based on two characters: a divided centropostzygapophyseal lamina in posterior dorsal vertebrae; and the presence of diapophyseal laminae on anterior caudal vertebrae. Neither character could be confirmed here, as very little of the neural arch below the diapophyses is preserved in the holotype specimen and both are scored ‘?’.

Salgado et al. (2006) recovered Amazonsaurus as a rebbachisaurid, but few synapomorphies listed for the group could be scored. Of four rebbachisaurid synapomorphies – lack of hyposphene-hypantrum in dorsal vertebrae, dorsal neural spines four times taller than centrum, dorsal and anterior caudal neural spines ‘petal’-shaped, and distally expanded scapula – only petal-shaped anterior caudal neural spines can be scored for Amazonsaurus . This analysis recovers this feature as a synapomorphy of a larger group, rather than Rebbachisauridae . Constraining Amazonsaurus to a position at the base of Rebbachisauridae incurs an additional parsimony debt of two steps.

Rauhut et al. (2005) recovered Amazonsaurus within Macronaria based on two characters: opisthocoelous posterior dorsal centra and forked chevrons restricted to the distal tail. The preserved dorsal centrum is slightly opisthocoelous and is presumably a mid- to posterior dorsal, although the cotyle is damaged. However, there are only eight preserved chevrons for Amazonsaurus , apparently consecutive, and based upon their size it is reasonable to assume they are anterior or anterior mid-caudal chevrons. The state of mid- and distal chevrons therefore cannot be determined. Re-running the analysis with only that single cell rescored as ‘?’ returns 855 equally parsimonious trees ( Table 7). A strict consensus of those trees results in a polytomy amongst the clades Flagellicaudata, Rebbachisauridae , Euhelopus + MDT, Omeisaurus + Cetiosauriscus , and all other taxa excluding Shunosaurus . The placement of Amazonsaurus as a macronarian in the analysis of Rauhut et al. (2005) is poorly supported, and its placement there cannot be distinguished from a placement basal to any of the above clades within Eusauropoda. Amazonsaurus is a basal diplodocoid in the current analysis, but the fragmentary nature of this taxon and the taxa immediately below and above it in the tree necessitate caution in any interpretation that relies on Amazonsaurus occupying this position. Templeton tests are unable to reject any of the previously published hypotheses of relationship based on the current dataset (P> 0.10).